Halogen-containing bisphenol A epoxy resins having blended therein aromatic polyamines, aliphatic polyamines, polyamide amines, amine adducts, dicyandiamide, acid anhydrides, phenol novolak resins and other curing agents have been used as adhesives, coating compositions, molding compositions, casting compositions and other compositions. It is also well known to dilute such blends with solvents to form varnish and apply the varnish to reinforcements by impregnation or coating. Laminates are molded using the impregnated reinforcements.
As the demand for size reduction and precision in the electric and electronic fields increases in the recent years, more severe requirements of heat resistance are imposed on adhesives, insulating coatings, encapsulants, laminates, and other materials for electronic parts in order to provide reliability during high-temperature service. Unfortunately, adhesives, coatings, encapsulants, laminates, and other materials prepared using commercially available conventional bisphenol A epoxy resins generally have a low heat distortion temperature and low electrical insulation and thus suffer from a loss of reliability.
The materials used in the electric and electronic fields are also required to have high flame retardancy. For example, known flame-retardant epoxy resins used as laminates (e.g., printed circuit boards prepared by laminating glass cloth and epoxy resin) include bisphenol A epoxy resins, typically a linear epoxy resin obtained by reacting a liquid bisphenol A epoxy resin having an epoxy equivalent of about 190 with tetrabromobisphenol A. When this linear epoxy resin is cured with dicyandiamide which is a versatile curing agent often used in laminate molding, for example, there is obtained a cured product (typically having a bromo content of 20 to 22% by weight) having a glass transition temperature Tg as low as 120.degree. to 130.degree. C. The heat resistance of the cured product may be increased by adding a polyfunctional epoxy resin such as an ortho-cresol novolak epoxy resin and a phenol novolak epoxy resin to the above-mentioned epoxy resin blend. However, since the addition of a substantial amount of such a polyfunctional epoxy resin sacrifices flame retardancy and moldability, the amount ofpolyfunctional epoxy resin must be limited.
In the conventional well-known epoxy resin compositions, heat resistance and flame retardancy behave counter to each other as described above. There is not available an epoxy resin composition which can meet heat resistance and flame retardancy at the same time. In the electronic field where higher performance is always desired, there is a need for an epoxy resin composition having both ehat resistance and flame retardancy so that a cured product thereof may become more reliable at high temperatures.
The above-mentioned blends of polyfunctional epoxy resins such as ortho-cresol novolak epoxy resins and phenol novolak epoxy resins cure into products which have a high modulus of elasticity and poor mechanical properties so that they are hard, brittle and prone to cracking upon heat shocks.
Japanese Patent Application Kokai Nos. 3015/1988 and 264623/1988 disclose a specific trifunctional epoxy resin and an epoxy resin composition obtained by reacting the specific trifunctional epoxy resin with a halogenated bisphenol. The resin and composition have the problem that in order to impart sufficient flame retardancy, the resin and composition are inevitably increased in viscosity. The viscosity can be reduced at the sacrifice of heat resistance.
When laminates are formed from such polyfunctional epoxy resin blends, their high viscosity causes inefficient impregnation, bubble entrainment, and thin spots, resulting in laminates which are electrically less reliable, for example, with respect to electrical insulation. Another requirement for the laminates is water resistance. If laminates are less resistant against water, the laminates tend to develop. quality lowering defects known as "blisters," "peeling," and "measling" during soldering of electric and electronic parts.